Enzymatic deinking process with pH shift and addition of alkaline cellulase

ABSTRACT

Good deinking of waste paper can be obtained at high consistency by first pulping at high pH, then lowering the pH, adding alkaline cellulase and letting the cellulase act during continued pulping and/or maceration. The improved deinking results in higher whiteness and better cleanliness (fewer ink particles) in paper made from the treated pulp after separation of ink particles. This cellulase treatment results in better operation of certain equipment such as thickeners and in better drainage of the pulp during paper making.

TECHNICAL FIELD

The invention relates to an enzymatic process for deinking of wastepaper.

BACKGROUND ART

Waste paper, such as old newspapers and magazines, is an important rawmaterial for the pulp and paper industry. However, the reutilization ofwaste paper requires special treatment to permit reincorporation intovarious kinds of paper.

Firstly, the waste paper must be disintegrated in water to obtain asuspension of separated fibres. Two general types of equipment for thispurpose are well known in the pulp and paper industry: low-consistencypulpers operating below 5% consistency (i.e. dry substanceconcentration) and high-consistency pulpers operating above 8%consistency, typically at 10-20%. Due to the presence of dispersedprinting ink the pulp will be grey, and it is mostly used for productionof grey cardboard.

Another type of treatment consists in disintegrating printed waste paperin the presence of chemicals in order to remove the ink. This so-calleddeinking combines pulping with chemical action to release the inkparticles from the fibres. The treatment is carried out in a pulper atalkaline conditions, the most common deinking chemicals being sodiumhydroxide, sodium silicate, and hydrogen peroxide; a surfactant is alsoadded to facilitate the removal of ink. Deinking is generally made at30°-50° C. The pH will generally be above 9, and in case of deinking athigh consistency (above 8%) a pH above 10 is usually required forsatisfactory deinking. The high pH leads to some yellowing of the pulp;this is reduced by the addition of hydrogen peroxide.

After this ink-releasing step, the pulp is diluted and purified toremove various contaminants, and the ink particles are removed from thepulp by washing, flotation or a combination of these.

It is known to use alkaline cellulase to improve the deinking duringlow-consistency pulping. Thus, JP-A 59-9299 (Kao) describes the use ofalkaline Bacillus cellulase at 3-6% consistency and 40°-45° C. Theprocess comprises 20 minutes pulping followed by 60-90 minutesmaceration. Deinking chemicals were added at the beginning of thepulping, and cellulase and surfactant were added at the beginning ofpulping or at the beginning of maceration.

JP-A 63-59494 (Honshu Paper) describes the use of alkaline cellulasefrom Humicola insolens to improve deinking at 5% consistency and 50° C.The process comprises 3-5 minutes pulping followed by maceration for140-360 minutes. Deinking chemicals, surfactant and cellulase were addedbefore or during the maceration. The pH was 9.1.

EP 262,040 (Cellulose du Pin) and EP 351,655 (Cultor) describe the useof acidic cellulase and hemicellulase during pulping at 2-5% consistencyand pH 3-7 for a different purpose (improvement of drainage). Nodeinking was performed.

It is described in TAPPI Journal, June 1989 that the effect of cellulaseon pulp has a maximum at 3% consistency and decreases from 3 to 5%.

Use of cellulase during high-consistency pulping (above 8%) has not beendescribed. It is the object of the invention to provide an improvedprocess for deinking of waste paper using alkaline cellulase.

STATEMENT OF THE INVENTION

We have found that surprisingly good deinking can be obtained at highconsistency by first pulping at high pH, then lowering the pH, addingalkaline cellulase and letting the cellulase act during continuedpulping and/or maceration. The improved deinking results in higherwhiteness and better cleanliness (fewer ink particles) in paper madefrom the treated pulp after separation of ink particles. Surprisingly,we have also found that this cellulase treatment results in betteroperation of certain equipment such as thickeners and in better drainageof the pulp during paper making, as indicated by a lowerSchopper-Riegler number.

Accordingly, the invention provides an enzymatic process for deinking ofwaste paper, comprising the following sequential steps:

a) pulping said waste paper at a consistency above 8% in the presence ofdeinking chemicals at a pH above 9.5,

b) lowering the pH to 6-9.5 by addition of an acidifying agent andadding alkaline cellulase,

c) continuing the pulping and/or maceration at consistency above 8%,

d) separating ink particles from the pulp.

DETAILED DESCRIPTION OF THE INVENTION

Pulping and maceration

The consistency in steps a-c) is generally above 10%, typically 10-20%.

The duration of pulping in step a) is generally 5-30 minutes.

Step c) may comprise pulping (mechanical disintegration) or maceration(incubation with or without stirring) or a combination of these. Thetotal duration of step c) is generally 30-120 minutes.

After the high-consistency treatment in steps a-c), the pulp mayoptionally be diluted to 2-6% consistency and further macerated for15-120 minutes.

The temperature throughout the process may be 15°-60° C. typically30°-50° C.

Pulping equipment

A conventional high-consistency pulper, used in the paper industry, canbe used in practice of the invention. Pulpers equipped with a rotor orconical screw ("Helico" pulpers) from E et M Lamort, Black Clawson,Voith an other makers can be used at consistencies up to 18% (dependingon the type of pulp). At consistencies above 18%, a disk pulper ispreferred.

Ink separation

After the treatment in steps a-c) (and optionally further maceration atlow consistency), the ink particles have been released from the fibres.The ink may then be separated from the pulp by methods known in the art.Preferred methods comprise flotation, washing or a combination of these.

After ink separation, the deinked pulp is suitable for paper making byconventional methods.

Alkaline cellulase

The invention uses an alkaline cellulase which is active and stable inthe pH range 6-9.5, especially 7-9.5, and preferably one with pH optimumin this range. The invention is particularly advantageous when using acellulase with reduced activity above pH 10, one with less than 50%activity at pH 10.5 compared to pH 9. Microbial cellulases are preferredfor reasons of economy.

It is preferred to use cellulase derived from Humicola insolens(described in U.S. Pat. No. 4,435,307), available as Novozym 342 fromNovo Nordisk A/S. Other examples are alkaline cellulase derived fromBacillus (U.S. Pat. No. 3,844,890), Aeromonas (U.S. Pat. No. 3,983,002)or Pseudomonas.

The dosage of cellulase preferably corresponds to a cellulase activityat pH 9 of 250-5000 CEVU/kg paper dry matter.

Deinking chemicals

The amount of deinking chemicals used in step a) will generally be:0.6-1.5% of sodium hydroxide, 1-3% of sodium silicate and 0.6-1.5% ofhydrogen peroxide (% of waste paper dry matter). Further, a surfactant,e.g. a nonionic one, will usually be added. The pH in step a) willnormally be in the range 9.5-12, usually above 10, typically 10-11.

Acidification

The acidifying agent added in step b) can be a mineral acid such assulphuric acid (added in the form of diluted acid). Also a salt of astrong acid and a weak base can be used, e.g. aluminium sulphate, theproperties of which make it useful in the paper industry.

An organic acid (such as gluconic or citric) can also be used partly orentirely in order to adjust the balance between free calcium andsequestered calcium as needed to improve the flotation.

The pH after step b) is preferably 7-9, especially 7.5-8.5, depending onthe properties of the cellulase used. In step b), the pH will usually belowered by at least 1 unit.

Determination of brightness according to the ISO standard

This is expressed in % as the ratio of diffuse reflection of the pulp todiffuse reflection of a perfect reflector, measured by reflection in thepurple to blue range centred at 457 nm (band width at half-height 44nm).

Determination of cleanliness

This is the number of dark points on a given area of a sample of papersheet. This may be counted visually or optico-electronically.

Determination of cellulase activity (CEVU)

A substrate solution is prepared, containing 33.3 g/l CMC (Hercules 7LFD) in 0.1M Tris buffer at pH 9.0. The enzyme sample to be analyzed isdissolved in the same buffer. 10 ml substrate solution and 0.5 ml enzymesolution are mixed and transferred to a viscosimeter (e.g. Haake VT 181,NV sensor, 181 rpm) thermostated at 40° C. One Cellulase Viscosity Unit(CEVU) is defined as the amount of enzyme that reduces the viscosity toone half under these conditions.

Determination of pulp drainage (Schopper-Riegler)

The Schopper-Riegler number (SR) is determined according to ISO standard5267 (part 1 ), on a homogenous pulp at a consistency of 2 g/l. A knownvolume of pulp is allowed to drain through a metal sieve into a funnel.This funnel has an axial hole and a side hole. The volume of filtratethat has passed through the side hole is measured in a special vesselgraduated in Schopper-Riegler units.

EXAMPLE 1 (Reference)

Three different, homogenous samples of waste papers are used incomparison tests aiming at measuring the brightness of pulps deinked bytwo different methods. The first deinking method is a conventionalchemical treatment. The second deinking method is a chemi-enzymatictreatment (named: "simultaneous enzymatic treatment") not according tothe invention. The latter is characterized by the addition of thecellulase/hemicellulase enzyme preparation during paper pulping, 5minutes after the beginning of the pulping. The enzyme action is"simultaneous" to the action of the chemicals.

Trials are realized in a 20 liters laboratory high consistency pulper,manufactured by E & M Lamort. Following pulping and maceration, the pulpis diluted to 0.8% consistency and subjected to a flotation step in a 17liters laboratory batch flotation cell.

The results of the trials on old newspapers (Once read news, B1 qualityin the European standard quality list edited by CEPAC), on high gradepapers (Coloured woodfree shavings, C2), on waste magazines (Mixed pamsand magazines, A6) are respectively shown in Tables 1a through 1c. Itcan be seen that the simultaneous enzyme action provides 2 to 3 pointsISO brightness gains compared to the chemical treatment.

EXAMPLE 2

In this series of trials, three deinking methods were compared, based onthe brightness of the pulps deinked: (1) by a chemical treatment, (2) bya simultaneous chemi-enzymatic treatment as described in Example 1, (3)by a "sequential enzymatic treatment" according to the invention, thelatter being characterized by a pH shift during the pulping, prior toenzyme introduction into the pulp.

The waste paper used in these experiments is a well-defined, homogenousmixture of 50% high grade papers (C2) and 50% newspapers (B1). Thepulping and the flotation are operated in the same conditions as inExample 1. The results are shown in Tables 2a (comparison of thechemical and the simultaneous treatment) and 2b (comparison of thechemical and the sequential treatment).

In trials 2a and 2b the control pulps (chemical treatment) havebrightnesses ranging from 65 to 66.5 following deinking (the differencebetween 65 and 66.5 is hardly significant). When using 5 l/t enzyme in asimultaneous treatment, the brightness is enhanced by 2 to 3.5 points.This confirms Example 1 results. However, when a sequential treatment isused (2b), the brightness increase is 5.2 points compared to the controlexperiment. The sequential enzymatic treatment also provides morebrightness gain than the simultaneous treatment with 2.5-fold lessenzyme (2 l/t instead of 5 l/t).

EXAMPLE 3

Two deinking trials are realized with printed shavings (C2) as the rawmaterial. In trial 3a (control), the waste papers are pulped at 14%consistency in a 20 liters pulper in the presence of deinking chemicals:1% (based on dry pulp) sodium hydroxide, 2.5% sodium silicate, 1%hydrogen peroxide, 0.5% soap (Olinor 4010, Henkel-Nopco). Pulpingconditions are pH 10.6 45° C./12 min. The pulp pH is then shifted to8.5. with diluted sulfuric acid. The pulp is then diluted to 4%consistency and incubated at 50° C. for 45 minutes. The SR number ismeasured following this maceration, and the pulp is diluted to 0.8%consistency and subjected to flotation in a 17 liters batch flotationcell. In trial 3b the procedure is the same, except that the enzymepreparation Novozym® 342 (545 CEVU/g) is added to the pulp following thepH shift, at the dose of 4 liters per ton of waste papers.

The results in Table 3 show that the enzyme provides a 13% SR decreaseand a 3 points brightness increase.

EXAMPLE 4

The trials described in this example were realized in a pilot deinkingplant (E & M Lamort) comprising: a high consistency helico pulper, ascreening machine for the removal of big size impurities, a macerationtank, a micro-vortex CH cleaner, two flotation cells in a series,hydro-cyclon cleaners and an SP cleaner. The deinked pulp coming out ofthis equipment can be washed on a curved-screen sive. Five trials wererealized in this installation with a homogenous sample of printedshavings (C2). Each trial was conducted with about 200 kg of these wastepapers.

Trial no. 1 (reference): chemical deinking with 1% (based on dry pulp)sodium hydroxide, 2.5% sodium silicate, 1% hydrogen peroxide, 0.5% soap(Serfax MT90, Stephanson Bros Ltd.).

Trial no. 2 (reference): simultaneous chemi-enzymatic deinking.Chemicals dosed as in no. 1, and Novozym® 342 (545 CEVU/g) was addedafter the chemicals, at the dose of 4 l/t waste papers.

Trial no. 3 (reference): chemical deinking as no. 1, but pH was shiftedto 8.5 with diluted sulfuric acid following 5 min pulping.

Trial no. 4 (invention): sequential chemi-enzymatic deinking. Chemicalsdosed as in no. 1, pH shifted to 8.5 as in no. 3, and Novozym® 342 (545CEVU/g) was added after the pH shift at the dose of 4 l/t waste papers.

Trial no. 5 (invention): same as no. 4, in order to check therepeatability.

In each trial the brightness of the pulp (Table 4b) was measured afterthe pulping step (5 sheets per trial), at the outlet of the flotationcells (4 sheets per trial), at the oulet of the SP cleaner (4 sheets pertrial), and at the oulet of the curved-screen sieve (2 sheets pertrial). The temperature, the pH and the consistency were measured atseveral places during each trial in order to ensure that the plant didwork well. These values are shown in Table 4a.

The brightness gains provided by the use of the sequentialchemi-enzymatic deinking are reproducible and significant at eachprocess step. In trials nr. 3, 4 and 5 the brightness of the pulp at thepulper outlet is lower than in trials 1 and 2. This has been caused by achange in ink colour during pH shift; it is not related to a lowerflotation efficacy, as seen in brightness values following flotation.When comparing trials 1 and 3, it seems that the flotation works betterwhen the pH has been lowered during the pulping.

The brightness gain following flotation is around 5 points, and 3 pointsfollowing cleaning. It can be seen that the washing step (curved-screensieve) causes in all trials a significant brightness increase due to theremoval of fine particles. However, the gap between the controlexperiments and the sequential treatment still exists (4 points), whichproves that this process is compatible with either flotation or washingtechniques (or a combination of the two).

                                      TABLE 1                                     __________________________________________________________________________    Comparison between the chemical and the simultaneous treatments               (reference)                                                                           Raw material                                                                  1a        1b            1c                                                    Newspaper (B1)                                                                          High grade (C2)                                                                             Magazines (A6)                                             Enz.        Enz.          Enz.                                   Treatment                                                                             Chemical                                                                           simultan.                                                                          Chemical                                                                             simultan.                                                                            Chemical                                                                             simultan.                              __________________________________________________________________________    Chemicals (a)                                                                         1/2.5/1/1                                                                          1/2.5/1/1                                                                          0.6/1.2/1/0.5                                                                        0.6/1.2/1/05                                                                         0.6/1.2/1/0.5                                                                        0.6/1.2/1/0.5                          Pulping (b)                                                                           8/45/15                                                                            8/45/15                                                                            10/45/15                                                                             10/45/15                                                                             10/45/15                                                                             10/45/15                               pH       9.9  9.9  9.3    9.3    9.1    9.1                                   Enzyme (c)                                                                            0    5    0      2      0      2                                      Maceration (d)                                                                        8/40/90                                                                            8/40/90                                                                            10/40/45                                                                             10/40/45                                                                             10/40/45                                                                             10/40/45                               Final Iso                                                                             57.5 60.5 76.5   79.5   60.0   62.0                                   brightness (e)                                                                __________________________________________________________________________     (a): chemicals doses, % based on dry pulp, respectively: sodium               hydroxide/sodium silicate/hydrogen peroxide/soap (Serfax MT90)                (b): pulping parameters, respectively: consistency (%)/temperature            (°C.)/time (min)                                                       (c): enzyme = Novozym ® 342, activity 1310 CEVU/g (l/ton waste papers     (d): maceration parameters, respectively: consistency (%)/temperature         (°C.)/time (min)                                                       (e): brightness of the deinked pulp, measured at 457 nm (Photovolt 577        apparatus) on filter made sheets                                         

                  TABLE 2                                                         ______________________________________                                        Comparison between the chemical,                                              the simultaneous and the sequential treatments                                        2a           2b                                                                           Enz.             Enz.                                               Chemical  simultan.                                                                              Chemical                                                                              sequential                               Treatment (ref.)    (ref.)   (ref.)  (inven.)                                 ______________________________________                                        Chemicals (a)                                                                           1/2.5/1/1 1/2.5/1/1                                                                              1/2.5/1/1                                                                             1/2.5/1/1                                Pulping (b)                                                                             8/45/15   8/45/15  8/45/15 8/45/15                                  Enzyme (c)                                                                              0         5        0       2                                        Maceration (d)                                                                          9/40/90   8/40/90  8/40/45 8/40/45                                  pH shift  no        no       no      7.5                                      Final ISO 65.0      68.5     66.5    71.7                                     brightness (e)                                                                ______________________________________                                         (a): chemicals doses, % based on dry pulp, respectively: sodium               hydroxide/sodium silicate/hydrogen peroxide/soap (Serfax MT90)                (b): pulping parameters, respectively: consistency (%)/temperature            (°C.)/time (min)                                                       (c): enzyme = Novozym ® 342, 1310 CEVU/g (l/ton waste papers)             (d): maceration parameters, respectively: consistency (%)/temperature         (°C.)/time (min)                                                       (e): brightness of the deinked pulp, measured at 457 nm (Photovolt 577        apparatus) on filter made sheets                                         

                  TABLE 3                                                         ______________________________________                                        Effect of the alkaline cellulase on SR and brightness                                         3a     3b                                                                     Control                                                                              Invention                                              ______________________________________                                        Pulping (a)       14/12    14/12                                              pH shift          8.5      8.5                                                Novozym ® 342 (l/t)                                                                         0        4                                                  SR                31       27                                                 Brightness (b)    75.5     78.5                                               ______________________________________                                         (a): consistency (%)/time (min)                                               (b): ISO brightness following flotation                                  

                                      TABLE 4a                                    __________________________________________________________________________    Pilot plant parameters                                                             Pulper   T macer-                                                                           pH  T    pH   T    pH   Pulper                                                                             Flotation                                                                          SP   Sieve                    °C./                                                                       pH   ation                                                                              macer-                                                                            flotation                                                                          flotation                                                                          flotation                                                                          flotation                                                                          con- con- con- con-                Trial no.                                                                          min.                                                                              pulper                                                                             °C./min.                                                                    ation                                                                             1 (°C.)                                                                     1    2 (°C.)                                                                     2    sistency                                                                           sistency                                                                           sistency                                                                           sistency            __________________________________________________________________________    1    42/12                                                                             10.6 33/45                                                                              10.0                                                                              37   8.6  39   8.2  14.2%                                                                              1.05%                                                                              0.68%                                                                              1.22%               2    42/12                                                                             10.6 35/45                                                                              10.0                                                                              37   8.6  39   8.8  15.2%                                                                              0.88%                                                                              0.81%                                                                              1.22%               3    42/12                                                                             10.6-8.3                                                                           34/45                                                                              7.7 38   7.6  36   7.7  14.3%                                                                              0.98%                                                                              0.70%                                                                              1.40%               4    42/12                                                                             10.6-8.4                                                                           29/45                                                                              7.8 38   7.6  37   7.7  14.5%                                                                              0.63%                                                                              0.48%                                                                              1.24%               5    43/12                                                                             10.6-8.4                                                                           36/45                                                                              7.6 38   7.6  39   7.8  14.0%                                                                              1.27%                                                                              0.45%                                                                              1.22%               __________________________________________________________________________

                  TABLE 4b                                                        ______________________________________                                        Pilot plant results (brightness)                                                      Pulper                                                                              Flotation    SP     Sieve                                       ______________________________________                                        Average                                                                       Trial no.                                                                     1         61.1    75.4         79.1 84.6                                      2         60.8    74.5         79.5 85.5                                      3         56.7    76.1         79.8 84.3                                      4         55.5    80.0         82.7 88.4                                      5         56.2    80.1         81.1 87.7                                      Standard                                                                      deviation                                                                     Trial no.                                                                     1         0.95    0.40         0.78 0.00                                      2         0.97    0.46         0.65 0.72                                      3         0.85    0.81         0.64 0.37                                      4         1.26    0.59         0.52 0.52                                      5         1.25    0.77         0.56 0.70                                      ______________________________________                                    

We claim:
 1. An enzymatic process for deinking of waste paper,comprising the following sequential steps:a) pulping said waste paper ata consistency above 8% in the presence of deinking chemicals at a pHabove 9.5, b) lowering the pH to 6-9.5 by addition of an acidifyingagent and adding alkaline cellulase, c) continuing the pulping and/ormacerating at a consistency above 8%, separating ink particles from thepulp.
 2. The process according to claim 1, wherein the consistency instep (a) is above 10%.
 3. The process according to claim 1, wherein theconsistency in step (a) is 10-20%.
 4. The process according to claim 1,wherein the duration of step (a) is 5-30 minutes.
 5. The processaccording to claim 1, wherein the pH in step (a) is in the range 9.5-12.6. The process according to claim 1, wherein the pH in step (a) is inthe range 10-11.
 7. The process according to claim 1, wherein the pH instep (b) is in the range 7-9.
 8. The process according to claim 1,wherein the pH in step (b) is in the range 7.5-8.5.
 9. The processaccording to claim 1, wherein the pH adjustment in step (b) is made withsulphuric acid, aluminum sulphate or an organic acid.
 10. The processaccording to claim 9, wherein the organic acid is citric or gluconicacid.
 11. The process according to claim 1, wherein the amount ofcellulase added in step (b) corresponds to 250-5000 CEVU/kg of dry wastepaper.
 12. The process according to claim 1, further comprising additionof a hemicellulase in step (b).
 13. The process according to claim 1,wherein the alkaline cellulase added in step (b) is derived from astrain of Humicola.
 14. The process according to claim 1, wherein thealkaline cellulase is derived from a strain of H. insolens.
 15. Theprocess according to claim 1, wherein the duration of step (c) is 30-120minutes.
 16. The process according to claim 1, further comprisingdilution of the pulp after step (c) to a consistency of 2-6% andmaceration of the pulp for 15-120 minutes.
 17. The process according toclaim 1, wherein the separation of the ink particles in step (d) isperformed by flotation or washing, or a combination of these.